1. Field of the Invention
The present invention relates to a process for the production of a shaped article of a heat resistant polymer. More particularly, the invention relates to a process for the preparation of a shaped article of an insoluble, infusible, heat resistant polymer which comprises melt extruding a polymer containing in the polymer chain at least one heterocyclic ring per repeating unit or a polymer wherein a part of said polymer is a precursor of a heterocyclic ring, with a small amount of a solvent to shape said polymer and thereafter heat treating the shaped polymer.
2. Description of the Prior Art
The preparation of a shaped article of heat resistant polymer containing a heterocyclic ring in its molecular structure has hitherto been accomplished by means of a dry shaping process, as follows:
In general, heat resistant polymers having completely cyclized heterocyclic rings are insoluble and infusible and many of them cannot be even dry-shaped. Therefore, it is conventional, for the purpose of shaping a heat resistant polymer, to first prepare a non-ring-closed polymer having such a structure that it is capable of forming a heterocyclic ring, and which is soluble in a specified organic solvent, then to prepare a solution of the non-ring-closed polymer in such solvent, dry-shaped this solution and thereafter subject the dry-shaped solution to heat treatment in a manner to close the intramolecular ring to produce the desired shaped article as a heat resistant polymer.
The steps of a typical dry-shaping process are generally shown as follows: ##SPC1##
However, there are various technical deficiencies in such dry-shaping processes. For example, when the flowed film is deposited upon a heated drum to remove the solvent by drying, voids tend to be created in the resulting film unless the heating and drying conditions are very carefully controlled. Further, after the heating and drying step, it is extremely difficult to peel the film off the heated drum, with the result that the film is sometimes broken, because of the strong adhesion that exists between the surface of the drum and the film. Also, since it is necessary to use a relatively large amount of solvent in this process, there are various problems brought about in removing and recovering the solvent, for example, the necessary equipment is complicated and expensive and the process is slow.
As prior art, U.S. Pat. No. 3,428,602 describes a process involving flowing a dilute solution of polyamide acid into a thin film, removing the solvent to some extent, shaping the thin film into a thick film by calender rolls and finally heat curing the thick film to obtain a thick shaped article of polyimide. Because this process also uses a solution of polyamide acid containing a large amount of the solvent, this process is accompanied with deficiencies similar to those heretofore discussed. Further, there are other deficiencies such as the difficulty of obtaining an article having the shape of a uniform plane, because shaping is carried out using rolls, and because deterioration of the physical properties of the material is inevitable as the thin film is heated and shaped by means of rolls.
Accordingly, an object of the present invention is to eliminate the aforementioned deficiencies of the conventional process and to provide a superior process for the preparation of a shaped article of a heat resistant polymer containing a heterocyclic ring which becomes ultimately insoluble and non-meltable.